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Why do planets have gravity?

03 September 2013

Gravitation (or gravity), quite a fascinating phenomenon, isn’t it? It holds everything down here. It even keeps the Moon from flying away and the satellites sent by man. It also keeps us from flying away as well! But how and why? How does such a phenomenon holds such massive celestial bodies together as a system (solar system)? Why do planets even have gravity? This question is probably asked throughout the world by students, including me. So let’s find out.

Earth's gravity measured by NASA's GRACE mission. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker.

Earth’s gravity measured by NASA’s GRACE mission. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker. By NASA/JPL/University of Texas Center for Space Research.

Gravity is one of the four fundamental forces of nature (with the weak force, strong force, and electromagnetism), but it’s also one of the most resistant to a proper, complete, and satisfying explanation. In the 17th century, Galileo Galilei put the first piece of the gravity “puzzle” in place when he discovered that, if you take air resistance out of the equation, all objects fall the at the same rate. Some 60 years later, Isaac Newton realised that this same force is what keeps the Moon in orbit around the Earth and the planets around the Sun, and was also responsible for the mysterious tides. He proposed that if you have a large mass such as the Earth, then it acts on any other mass in such a way that a force between the two of them which is proportional to the product of their masses and inversely proportional to the square of their distance.

Since we’re talking about Isaac Newton, I’d like to debunk the “apple rumour.” The rumour that an apple dropped on his head is just gibberish. What actually happened, as Hawking and deGrasse Tyson say, is that he was inside his house staring outside the window in a contemplatively. He noticed an apple fall from the tree in front of his house and saw the Moon in the sky. That made him question that if an apple is falling down, why isn’t the Moon also falling towards us?

Newton’s tremendous achievement, with the astronomical data and Kepler’s inferences, describes gravity’s behaviour; but fails to answer why gravity exists in the first place. It also does not say that the strength of gravitation depends on how much mass there is and how far away you are from this mass.
Then comes Albert Einstein with his general theory of relativity. Personally, my favourite topic in physics!

Einstein’s interpretation of gravity

Newton saw gravity as an instantaneous influence. if you have a mass, it instantaneously affects the motion of other masses.

Einstein, in the early 20th century, saw this as a flaw in Newton’s theory and inserted a new piece in the gravitational puzzle — time. His work, the Special Theory of Relativity, is that the speed of light is a universal speed limit so that nothing happens instantaneously; everything takes time. With this, Einstein concluded that clocks tick at differing rates and the lengths of meter sticks will differ depending on whether you are on Earth, on a satellite orbiting the Earth, or elsewhere in the Universe. His new view enabled scientists to map gravitational forces using a curved space/time grid.

Space-time curvature Earth's mass curving space-time.

Space-time curvature
Earth’s mass (gravity) curving space-time.

Einstein’s theory of gravity works in more cases than Newton’s. His model, the general theory of relativity, which is a theory of gravity, space and time, predicts that light will bend in a gravitational field. His theory also predicts the existence of black holes itself; they have such strong gravitational pull that even light cannot escape, resulting in the colour black. The interpretation that Einstein provides is that there is no “force” of gravity at all, but rather space and time are bent in such a way that a particle moving freely with no other forces on it will follow a path that bends along with the local geometry of space-time.

To understand this, you might have to imagine a bowl and the Sun at the bottom. Leave a ball (suppose this is Earth) on the top. What will happen? The Earth will spiral down and circle (or revolve) around the sun until it crashes with the Sun. The same thing is happening in our solar system. Our Earth is actually falling towards the Sun but it will billions of years for us to “crash” into the Sun. That’s how revolution works in space. Even the Moon is falling upon us!

Both, Einstein’s and Newton’s theory of gravity have the fundamental feature that mass creates the effects of gravity but do not explain why, and why there are no other sources of gravity. For instance, other forces of nature depend on certain types of “charges” — like electrical charge, to generate the fields and react to them. Gravity’s “charge” is simply matter and energy.
So in average person terms, anything that has mass has gravity. The more mass something has (the bigger it is), the more gravity it will have. So everything around us have gravity. Even me and you have gravity; it’s just that people are so small that there is not enough gravity to even matter. Even the biggest building on Earth is too small to matter. But since planets are really big, they have enough gravity to hold things down. Well, then why does mass have (or create) gravity?

How does mass create gravity?

Literally every mass – from an apple to the Sun – distorts space and time. Gravity is the manifestation of that distortion. While Newton’s formulation is accurate enough to plan space missions across the Solar System, Einstein’s refinements are required not only to explain a number of very precisely measured astronomical phenomena, but are also at the heart of our modern understanding of the history of the universe.

To be honest, we don’t exactly yet know how gravity is carried, but there are researches going into gravitational waves.
Force due to gravity is related by:

fgmmr2

Where G is a constant, M is the mass of the first body, m is the body of the second body, and r is the distance between them.

Even things as small as electrons and quarks have mass, but because they have so little mass, the gravitational force is tiny, and for most calculations can be forgotten about. On the universe size scale, electromagnetism, the weak and the strong force are ineffective. Gravity is significant, especially on heavy bodies such as planets and stars. The basic answer is that mass is what makes gravity, but we are not sure how. Mass can be thought of in general relativity as a “gravitational charge,” like an electron can be though of as an electric charge. This is called the weak equivalence principle.

As for what actually causes gravity to exist, again, we don’t know, there are theories like a particle called a Graviton could carry the force of gravity. Higgs boson is one of the famous theories to be the cause of mass and is being looked for at the Large Hadron Collider. Other people think that gravity is just a consequence of spacetime being curved by mass.

References & Resources

  1. Ask The Van, Department of Physics, University of Illinois. Retrieved from:
    http://bit.ly/14mTjYG
  2. Riemenschneider, P., “How does a mass create gravity?”, Driven to Discover. Retrieved from:
    http://bit.ly/1dKzZOC
  3. Yahoo! Answers
  4. Gravitation, Wikipedia
  5. Graviton, Wikipedia
  6. General relativity, Wikipedia
  7. Newton’s law of universal gravitation, Wikipedia
  8. Images via Wikimedia Commons.

Atul Anand Sinha - post author

I love understanding and writing about the universe, especially about its physical nature and its relation to mathematics. I also am a recreational programmer.

Please don't hesitate to contact me at my work: [email protected]

  • http://atul.asdfscience.com/ Atul Anand Sinha

    Yes! Thanks a lot for posting this video. It relatively explains better than the depressed-sheet analogy. :)